In recent years, X-ray imaging apparatus often use a subtraction technique which carries out an image subtraction process on two X-ray images of the same site picked up with different photographing conditions, thereby to obtain a new subtraction image.
A specific difference factor of photographing conditions between the two X-ray images may be energy intensity of X-ray beams irradiating the object under examination. In the case of a subtraction technique of the dual energy mode using two types of X-ray beams, i.e. with high energy and low energy, a high-energy X-ray image is obtained from the X-ray beam of high energy, and a low-energy X-ray image from the X-ray beam of low energy, and thereafter a subtraction image is obtained by carrying out a subtraction between the high-energy X-ray image and low-energy X-ray image based on predetermined weights applied to the high- and low-energy X-ray images. Further, by suitably adjusting the level of weighting set beforehand to the X-ray images, it is possible to obtain, as a subtraction image, a bone image from which soft tissue is eliminated, or, conversely, a soft tissue image from which bones are eliminated (see Patent Document 1).
A specific difference factor of photographing conditions between the two X-ray images may be whether or not a contrast medium is injected into the object under examination. In the case of a subtraction technique of the contrast medium injection mode accompanied by injection of a contrast medium into the object under examination, X-ray images are obtained before injection of the contrast medium, and X-ray images after injection of the contrast medium, and thereafter subtraction is carried out to obtain subtraction images selecting a site into which the contrast medium has been injected. In the case of angiography in which a contrast medium is injected into blood vessels, an X-ray imaging apparatus carries out a subtraction after obtaining X-ray images before injecting the contrast medium into the blood vessels, and X-ray images after injecting the contrast medium into the blood vessels, to obtain subtraction images selecting, as the site of interest, the blood vessels into which the contrast medium has been injected.
In addition, a conventional X-ray imaging apparatus using the subtraction technique of the contrast medium injection mode carries out a 3D reconstruction for obtaining a 3D image showing blood vessels which are a contrast medium injection site, based on a plurality of subtraction images of the same photographic site picked up from different photographing directions. Specifically, before injecting the contrast medium into the blood vessels, X-ray images in different revolutional phases are successively obtained while revolving an X-ray tube for X-ray emission and a two-dimensional X-ray detector for transmitted X-ray image detection around the patient. Next, after injecting the contrast medium into the blood vessels, as before the contrast medium injection, X-ray images in different revolutional phases are successively obtained while revolving the X-ray tube for X-ray emission and the two-dimensional X-ray detector for transmitted X-ray image detection around the patient.
Then, a subtraction image is obtained for each revolutional phase by carrying out an image subtraction process for two X-ray images picked up before and after contrast medium injection, in the same revolutional phase of the X-ray tube and two-dimensional X-ray detector. Further, a 3D reconstruction process is carried out based on pixel signals of a large number of subtraction images obtained, and revolutional phases corresponding to geometric positions of the respective subtraction images, thereby obtaining a 3D X-ray image selectively showing the blood vessels in the patient as a site of interest. The 3D X-ray image of the blood vessels obtained in this way is used for checking the state of flow through the blood vessels or determination of a treatment policy.
[Patent Document 1]
Unexamined Patent Publication H11-188024 (Page 6, Col. 7 and FIG. 1)